VK API method why photos.getMessagesUploadServer displays an error 15?

There is a VK app Iframe.

From the backend refer to method photos.getMessagesUploadServer and constantly displays an error 15: Access denied: no access to call this method, although I request access_token from GET-request to the frame. Similarly, the appeal to method photos.getWallUploadServer — everything is okay.
July 2nd 19 at 13:10
1 answer
July 2nd 19 at 13:12
All because
This method can be called with the access key of the user obtained in the Standalone-application using Implicit Flow.
as it is impossible to work with messages in iframe applications.
And photos.getWallUploadServer possible, done on a wall can be sent from an iframe
Thank you, I understand :) - Lon_Altenwerth commented on July 2nd 19 at 13:15

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